Localized control of integrated circuit parameters using focus ion beam irradiation

ABSTRACT

A method of localized control of integrated circuit parameters according to the present invention is used to adjust a the threshold voltage of an integrated circuit by irradiating an inoperable area with a focused ion beam such that the determination of the correct threshold voltage is facilitated without having to refabricate the integrated circuit in its entirety.

FIELD OF INVENTION

The present invention relates to manufacture of integrated circuits. Inparticular, the present invention is directed towards localizedparameter control of integrated circuits using focused ion beamirradiation.

BACKGROUND OF THE INVENTION

One of the most widely used electronic devices especially in digitalintegrated circuits ("ICs") is the metal-oxide semiconductor ("MOS")transistors. MOS transistors are characteristic in that the gateelectrode of a MOS transistor is a metal such as aluminum (Al) and theelectrode is isolated from the channel by an insulator such as siliconoxide (SiO₂). One of the most important parameters associated with MOStype transistor is the threshold voltage ("V_(T) "). V_(T) is thevoltage needed to turn on the transistor. That is to say, V_(T) is theminimum voltage at which electrons begin to move from the source of thetransistor to the drain, allowing the transistor to conduct electricity.

The current process by which ICs are developed and produced bymanufacturers is shown in the flow diagram of FIG. 1. As shown, an IC isinitially designed in step 10 with expected V_(T) values obtained duringthe design stage of the device. Then in steps 12-16, V_(T) adjustmentprocessing is performed on a wafer using various known methods such asphotoresist patterning, ion implantation, and heat treatment to set theV_(T) obtained during the initial design of the device. Typically, steps12-16 can take anywhere from one to three months to complete because ofthe time consuming processes involved in creating the wafer, the mask,and the adjusting procedure.

Once the V_(T) adjusting process is complete, a diagnostic test isperformed in step 18 to determine whether the operating parameters ofthe processed wafer meet specified requirements. Specifically, testingis performed on each area of the processed wafer to determine if theV_(T) values initially obtained during the design stage work on actuallyfabricated ICs. If the test fails on any part of the wafer, new valuesof V_(T) are calculated as shown in step 20 for the failed areas.Consequently, new V_(T) adjustment masks are designed and produced asshown in step 22, and the whole process begins again using the new maskwith a new wafer. Each time a V_(T) adjustment has to be made, theentire process as shown in FIG. 1 must be repeated until the processedwafer passes the test at which point the successful processingparameters are used to mass produce the designed device.

As one can imagine, this process of adjusting V_(T) is long, tedious,and very costly since each run must start with a new wafer and the runtakes anywhere from one to three months to complete. Hence, a simpler,faster, and less expensive method is needed in adjusting V_(T) values ofan IC device.

SUMMARY OF THE INVENTION

The present invention is directed to a faster, more efficient, and lessexpensive method of adjusting parameters of an IC. More specifically,the present invention is a method of localized control of IC parameters,such as V_(T) by using focused ion beam irradiation ("FIB").

Because of its ability to create resolution images, FIB technology isbeing used in the semiconductor arts as a developmental tool inobtaining precision views and cross-sectional views of semiconductorwafers for the purposes of physical defect detection. However, a sideaffect of using FIB irradiation for obtaining such an image is that theprocess changes the threshold voltage of the exposed area on the wafer.Hence, FIB scanners are used as destructive inspection tools for defectanalysis in semiconductor technology.

Ironically, this same side affect is harnessed according to the presentinvention to overcome the shortcomings of the prior art system asdescribed above. Therefore, a method of localized control of integratedcircuit parameters according to the present invention includes the stepsof:

(a) designing an integrated circuit with expected circuit parameters;

(b) processing the integrated circuit designed in step (a);

(c) performing a diagnostic test on the processed integrated circuit ofstep (b);

(d) if the diagnostic test fails, determining a new value of the circuitparameter;

(e) adjusting the circuit parameter by irradiating an area of theintegrated circuit that failed step (d) with focused ion beam; and

(f) repeating steps (c)-(d) until the integrated circuit passes thediagnostic test.

In particular, the controlled parameter is the threshold voltage of theintegrated circuit.

One advantage of the present invention is that the wafer processingprocedure does not have to be repeated every time V_(T) is adjusted.Instead, FIB irradiation is used to adjust the value of V_(T) in thefailed areas of the processed wafer until the correct V_(T) has beenset. Then, this V_(T) value is used in the mass production stage,reducing research and development time from one to three months typicalof the prior art system to just a few hours using the method of thepresent invention.

Another advantage of the present invention is that the parameters ofnon-critical IC devices, such as those used in toys or disposableequipment, can be adjusted using the method of the present invention andput directly to use without having to be refabricated using new wafers.This, in turn, reduces processing cost as well as manufacturing time,allowing vendors to pass the savings on to the consumers.

BRIEF DESCRIPTION OF DRAWINGS

The features and inventive aspects of the present invention will becomemore apparent upon reading the following detailed description, claims,and drawings, of which the following is a brief description:

FIG. 1 is a flow diagram describing a method of V_(T) adjustment used inthe prior art.

FIG. 2 is a flow diagram describing a method of V_(T) adjustmentaccording to the present invention.

FIG. 3 is a graph describing the threshold voltage in relation to thedosage of FIB irradiation.

FIG. 4 is a cross-sectional view of a semiconductor device irradiatedwith FIB.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2 shows a flow diagram describing the method of localized controlof IC parameters such as threshold voltage using FIB irradiationaccording to the present invention. As shown in step 30, an IC isdesigned with expected V_(T) values. Then, in step 32, a semiconductorwafer is processed according to the initial design obtained in step 30.When a new wafer has been processed, step 34 is skipped the first timethrough and the method goes directly to step 36. In step 36, theprocessed wafer is tested to confirm that each critical area of thewafer is operational. If the wafer passes the test, then the device issent to be mass produced. However, if the wafer fails the diagnostictest of step 36, a new value for V_(T) is determined in step 38 and sentto step 34.

In step 34, the new value of V_(T) is then used to apply a focused ionbeam on the failed area of the wafer with the correct amount of dosageto adjust the exposed area according to the newly determined V_(T)value. The wafer, then, is tested again to determine operability. Thisprocess of applying FIB irradiation, testing the wafer, and determininga new V_(T) value is repeated until the processed wafer passes thediagnostic test. The obtained V_(T) values are then used to create amask for mass production of the device.

The method of the present invention takes advantage of the fact thatthreshold voltage V_(T) is affected by exposure to FIB in a predictablemanner. As shown in FIG. 3, threshold voltage (V_(T)) is affected by thedosage (D) of FIB irradiated in a certain area of a semiconductor wafer.The equation describing the relationship between V_(T) and D is wellknown in the art. Preferably, the data describing the relationshipbetween V_(T) and D is obtained empirically for each piece of FIBequipment and stored in memory of the FIB controller. Furthermore, thedosage (D) is characterized by the equation: ##EQU1## where I: FIB beamcurrent

t: FIB irradiation time

A: FIB irradiation area.

Consequently, each of the variables I, t, and A may be customized for aparticular wafer and used in conjunction with the V_(T) -D data obtainedempirically to determine the dosage level needed to obtain a certainvalue of V_(T).

As an example, FIG. 4 shows a cross-section of a device on a processedwafer, e.g., a MOS transistor 40. When transistor 40 fails the test instep 36, i.e., the supplied V_(T) is not operable, a new V_(T) value iscalculated, and the new V_(T) value is then used to determine the dosageof the FIB using empirical data already collected for the FIB equipment.Then, using the dosage equation above, a duration of the exposure timeis determined since the current of the FIB beam and the FIB scan area 42is known. In this way, a new wafer does not have to be processed just todetermine the V_(T) of the inoperable transistor. Once the new value ofV_(T) is determined and the operability of the wafer verified, aproduction mask is developed and created based on the verified workingparameters to be used in mass producing the wafer. Accordingly, themethod of the present invention saves valuable time and cost associatedwith determining the correct working parameters of prototype waferdesign.

Alternatively, the method of the present invention may also be used toadjust wafers to be used directly in an application. Inoperable wafersdue to incorrect V_(T) values may be fixed by using the method of thepresent invention as described above thereby reducing the amount ofwafers normally thrown away. Having fully described the preferredembodiments of the invention, variations and modifications may beemployed without departing from the scope of the present invention.Accordingly, the following claims should be studied to learn the truescope of the present invention.

What is claimed is:
 1. A method of generating a production mask for massproduction of semiconductor devices including transistors, the methodcomprising:(a) designing an integrated circuit including MOS transistorswith expected circuit parameters including a threshold voltage parameterfor said transistors; (b) preparing the integrated circuit designed insaid designing step; (c) performing a diagnostic test on the integratedcircuit to determine an actual threshold voltage for one or more of saidtransistors; (d) comparing said actual threshold voltage with anexpected threshold voltage from said step of designing said integratedcircuit; (e) if said actual threshold voltage differs from said expectedthreshold voltage, adjusting said actual threshold voltage byirradiating selected portions of said integrated circuit with focusedion beam; and (f) when said actual threshold voltage is corrected inaccordance with said expected threshold voltage from said step ofdesigning said integrated circuit, preparing a mask for said integratedcircuit according to a design of said integrated circuit generated insaid step of designing said integrated circuit and according to saidadjustment made to said integrated circuit with said focused ion beam.2. The method of claim 1, further comprising mass producing integratedcircuits using said mask.
 3. The method of claim 1, further comprisingrepeating said steps of performing a diagnostic test, comparing saidactual threshold voltage to said expected threshold voltage andadjusting said actual threshold voltage using a focused ion beam untilsaid actual threshold voltage matches said expected threshold voltage.4. The method of claim 1, further comprising:determining a relationshipbetween change in actual threshold voltage and dosage of said focusedion beam irradiation for a particular ion beam irradiation device; andusing said relationship to control said step of irradiating selectedportions of said integrated circuit with said focused ion beam.
 5. Themethod of claim 4, wherein said determining a relationship for saidparticular ion beam irradiation device is performed using empirical datafrom previous operation of said ion beam irradiation device.
 6. A devicefor generating a production mask for mass production of semiconductordevices including transistors, the device comprising:(a) means forpreparing an integrated circuit according to a predetermined design,wherein said integrated circuit includes MOS transistors with expectedcircuit parameters including a threshold voltage parameter for saidtransistors; (b) means for performing a diagnostic test on theintegrated circuit to determine an actual threshold voltage for one ormore of said transistors; (c) means for comparing said actual thresholdvoltage with an expected threshold voltage determined according to saiddesign for said integrated circuit; (d) means for adjusting said actualthreshold voltage by irradiating selected portions of said integratedcircuit with focused ion beam if said actual threshold voltage differsfrom said expected threshold voltage; and (e) after said actualthreshold voltage is corrected in accordance with said expectedthreshold voltage, means for preparing a mask for said integratedcircuit according to said design of said integrated circuit andaccording to said adjustment made to said integrated circuit with saidfocused ion beam.
 7. The device of claim 6, further comprising means formass producing integrated circuits using said mask.
 8. The device ofclaim 6, further comprising:means for determining a relationship betweenchange in actual threshold voltage and dosage of said focused ion beamirradiation for a particular ion beam irradiation device; and means forusing said relationship to control said step of irradiating selectedportions of said integrated circuit with said focused ion beam.
 9. Thedevice of claim 8, further comprising means for gathering empirical datafrom previous operation of said ion beam irradiation device for use indetermining said relationship.